The present invention is directed generally to antennas. More particularly, the invention is directed to a spiral antenna having one or more voids wherein each void is adapted to receive another antenna.
Spiral antennas are known. See, for example, U.S. Pat. Nos. 2,856,605, 2,977,594 and 4,243,993. It is also known to combine two or more spiral antennas to form an antenna array. See U.S. Pat. Nos. 3,017,633 and 3,530,486. Still further it is known to combine spiral antennas having different bandwidths to form a broadband antenna system. See, for example, U.S. Pat. No. 4,095,230.
A problem with known broadband antenna systems is that multiple antennas cannot be combined to form a broadband antenna system or array and still fit within the space normally occupied by one of the antennas alone. Thus, replacement of a single antenna with a broadband antenna system or array often requires that the structure housing the original antenna be modified to provide additional space to receive the new antenna system.
The aforementioned problem often manifests itself in the replacement of antennas used in military aircraft. For example, radar warning receivers in military aircraft generally utilize a spiral antenna housed in a spaced provided in the external frame of the aircraft. Typically, the bandwidth of this spiral antenna is 2-18 GHz. Due to the recent appearance of radar signals above 18 GHz., it has become desirable to replace the existing spiral antennas with a broadband antenna system having an overall bandwidth of approximately 2-100 GHz. Known antenna systems covering this increased bandwidth generally comprise an array of spiral antennas which will not fit in the space currently provided for the existing spiral antenna. Therefore to install the new antenna system, it is necessary to modify the external frame of the aircraft by increasing the amount of space provided for the existing spiral antenna. This modification to the aircraft frame is undesirable because it is costly and time consuming.
It is therefore desirable to combine a number of antennas to form a broadband antenna system which occupies the space normally occupied by one of the antennas alone.